Sure thing! We've got 684 and we're gonna multiply it by 21. That's the same as adding 684 to itself 21 times.
Step 1: 0 + 684 = 684
Step 2: 684 + 684 = 1368
Step 3: 1368 + 684 = 2052
Step 4: 2052 + 684 = 2736
Step 5: 2736 + 684 = 3420
Step 6: 3420 + 684 = 4104
Step 7: 4104 + 684 = 4788
Step 8: 4788 + 684 = 5472
Step 9: 5472 + 684 = 6156
Step 10: 6156 + 684 = 6840
Step 11: 6840 + 684 = 7524
Step 12: 7524 + 684 = 8208
Step 13: 8208 + 684 = 8892
Step 14: 8892 + 684 = 9576
Step 15: 9576 + 684 = 10260
Step 16: 10260 + 684 = 10944
Step 17: 10944 + 684 = 11628
Step 18: 11628 + 684 = 12312
Step 19: 12312 + 684 = 12996
Step 20: 12996 + 684 = 13680
Step 21: 13680 + 684 = 14364

So, 684*21 = 14364